The present invention relates to a process for the nitration of aromatic organic compounds in the liquid phase, wherein the aromatic organic compound is intensively mixed with a mixture of nitric acid and sulfuric acid in a mixing nozzle, and fed to a reactor with active mixing. In a variation of this process, dinitrotoluene is produced from toluene.
Dinitrotoluene (DNT) is an intermediate from the production of toluene diisocyanate (TDI). DNT can be produced on an industrial scale by the nitration of toluene with nitric acid in the presence of sulfuric acid (see Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition, 2003, Wiley). The disadvantage of this process, however, is that the waste water that is obtained typically has to be cleaned before it can be discharged into surface water as waste water. It is relatively expensive to clean this waste water.
Accordingly, an object of the present invention is to reduce the formation of these secondary products that end up in the waste water, and hence, reduce the cost for cleaning the waste water.
The nitration of aromatics substantially takes place in the aqueous of the two phases. In order to react with one another, the components of the organic phase, such as, for example, toluene, must diffuse through the interface between the two phases to react in the aqueous phase in the presence of sulfuric acid with the nitric acid that is present there in excess. In such cases it is known that, depending on the reaction conditions, the effective speed of the reaction may depend greatly on the size of the phase interface. The size of this interface can be increased by intensive stirring, for example, which has an advantageous effect on the reaction speed. Other factors that influence the reaction speed include the reaction temperature, the concentrations in the organic phase of the components to be reacted and concentrations of the nitric acid and sulfuric acid in the aqueous phase. As the reaction proceeds, the water formed during nitration will dilute the sulfuric acid, thereby reducing the reaction speed. The reduction in the nitric acid concentration due to its consumption in the reaction also helps to reduce the reaction speed.
If the reaction is controlled appropriately, these dependencies can be used to control the nitration of the organic compounds in the organic phase but only to a certain level. For example, with an appropriate choice of temperature and sulfuric acid concentration in the waste acid, the nitration of toluene will proceed as far as mononitrotoluene (MNT) only, largely preventing the formation of DNT. In the same way, the formation of trinitrotoluene (TNT) can largely be prevented in the nitration of toluene to DNT.
It is also known that, under appropriate conditions, nitric acid is capable not only of nitrating but also of oxidizing the organic compounds. Possible compounds obtained by the oxidation of toluene, MNT or DNT are cresols, phenols and nitration products thereof. A low sulfuric acid concentration increases the oxidation tendency of nitric acid, so the content of organic secondary products in the reaction mixture will increase as the sulfuric acid concentration is reduced. This can lead to a lower limit of sulfuric acid concentration at which this process is economically viable.
Dinitrotoluene is conventionally produced in a continuous process, so the concentrations and temperatures in the reactors can largely be kept constant over time. Stirred-tank reactors or series of stirred-tank reactors, loop reactors and tubular reactors of various sizes, which in some cases have internal fittings, can be also used, for example. The thorough mixing that is required for the reaction can be achieved in various ways. If tubular reactors are used for the nitration, the use of mixing nozzles to achieve the thorough mixing is known and is described in, for example, EP-A-0373 966 or in DE-A-195 39 205. The mononitration of benzene that is described there, however, is not transferable to the dinitration of toluene. This is because in the nitration of toluene, the mixture obtained breaks down into the two phases again before the reaction is completed. For that reason, as described in, for example, EP-A-708 076, several of these mixing devices are used in series in the direction of flow. Although in the isothermal process, this leads to difficulties because of the heat exchangers that are needed to dissipate the reaction heat. For that reason, reactors with active mixing are used in the isothermal process. In the case of tubular reactors, circulating pumps can be used, in which case part or all of the recirculated stream is returned to the entrance of the reactor. In the case of stirred- tank reactors, as described in, for example, EP-A-903 336, thorough mixing is typically achieved through the use of a suitable agitator. The goal of having as selective a reaction as possible and widespread prevention of the formation of secondary products is not achieved through the use of the cited reactors alone, however.
Thus, the object of the present invention is to provide a simple and economic process for the nitration of organic compounds in which the formation of secondary products can largely be prevented.